Good point - that's the only issue I have with K9 and AGP on Android. I'm happy to live with only being able to view encrypted attachments on the desktop at the moment.
There's an open issue to add PPG/MIME: https://code.google.com/p/k9mail/issues/detail?id=5864
There's also an open bounty which can be contributed to: https://www.bountysource.com/issues/815255-pgp-mime